Most large computers operate on three phase AC power. For systems which consume more than 1.8 to 2.45 K voltAmps, the use of three phase AC power is often a utility company requirement. Three phase AC power systems also aid in the design of fault tolerant computers. When one phase is lost due to a building line disturbance or a circuit breaker trip, the computer system may continue limited operations using the remaining two phases.
In three phase power systems, it is desirable that the phasor sum of the three AC voltages be zero or, in other words, the source loading be balanced. This condition is often a utility company requirement. In the past, computer engineers relied upon the architecture or arrangement of the DC loads in the computer circuit to balance the load on the power source. The loads would be arranged such that the load on each phase of the power source would be equal. Thus to balance the load on the power source, it was necessary to balance the load itself. This requirement limits the combination and arrangement of computer memory and logic circuits available to the computer engineer in the design of the overall computer system.
Maintaining a balanced loading on the power source also reduces requirements on power distribution equipment design. When loading on the AC power source is unbalanced, a net, non-zero current flows in the neutral wire. Power distribution components such as transformers and wires must be adequately sized to handle this current as well as the phase currents. Typical three phase power systems generate current harmonics which impose current waveshape loadings that are neither sinusoidal nor substantially in phase with the voltage loading on the AC power source. As such, the power distribution system must be sized to accommodate these hiqh neutral currents, resultinq in increased costs for building power system components and less efficient energy utilization.
In addition to providing design flexibility and reduced component costs, it is also desirable that a computer power supply system be tolerant of power system faults and permit on line servicing of the computer. Computers form an integral part of the infrastructure of the modern world. The loss of a computer for servicing severely hampers the operational efficiency of the many businesses, banks, airlines, factories, etc. which utilize computers to conduct daily operations. As such it is desireable to produce a computer of modular construction such that a troublesome component can be removed for servicing without the need to shut down operation of the entire computer. In a computer system containing a power supply dependant upon the organization of components to maintain load balance, removing a component or a portion of the power supply for servicing results in an unbalanced load.